<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="utf-8"/>
        <title>01test</title>
        <script src="js/jquery-1.12.4.min.js"></script>
    </head>
    <body>
        <div id="box" class="wrap">test</div>
    </body>
    <script>
        /*$('#box').css('background','red'); */
       /* $('.wrap').css('background','red'); */
       $('div').css('background','blue');
    </script>
</html>